The radio astronomy group at the Arcetri Astrophysical Observatory is involved in the electromagnetic design of microwave devices for radio astronomy receiver front-ends. This activity focuses on the development of on-purpose custom microwave components, like feeds, orthomode transducers, polarizers and high-temperature superconductor filters. In this area, we have acquired experience in waveguide structures as well as in planar circuits ranging in the frequency band from tens of MHz up to 100 GHz.

Novel fabrication technologies, like 3D printing, electroforming and platelet, have been intensively studied aimed to performance improvement. Prototypes are usually characterized within the Arcetri radio lab both at room and cryogenic temperatures, the operating cold environment of most radio astronomical receivers.

Many of the front-ends installed at the radio telescopes operated by INAF are equipped with devices that have been designed, developed and tested at Arcetri.

Such know-how has been made available also in international collaborations for the development of modern instruments of worldwide interest for radio astronomy observations, like ALMA and SKA.

Huge efforts have been done in the successfully ALMA band 2 receiver front-end development, with the aim to extend the original bandwidth (69-90GHz) to a very broadband one (67-116 GHz), to cover with a single receiver also the original ALMA band 3 (84-116 GHz).

In the framework of the SKA project, the radio astronomy group at the Arcetri Astrophysical Observatory has been involved in several areas of the SKA-Low radio telescope. This instrument is based on low-frequency aperture array (LFAA) which is a modern technology for radio astronomers. The dual-polarized antenna model for SKA-Low adopted by the SKA System Critical Design Review is called “SKALA4.1” and it has been largely designed by a collaboration between INAF, CNR and Sirio Antenne. In 2019, in order to verify the antenna performance, 256 SKALA4.1 antennas have been built and deployed at the Murchison Radio-astronomy Observatory (MRO) in Western Australia.

Besides the design of the SKALA4.1 antenna, the group has been busy with the EM analysis to assess the performance of SKA-Low. This activity has included the analysis of the sensitivity and polarization performance of the SKA-Low station by making use of the Embedded Element Pattern (EEP). A deep understanding of the mutual coupling effects and on the EEP variability is a key-aspect to assure the instrumental calibratability of the array.

Experimental measurements on different array prototypes have been conducted by means of a Radio-Frequency source installed on a drone, which has been developed by CNR-IEIIT. The last UAV campaign was performed in June 2019 by INAF and CNR teams at MRO, where hundreds of EEPs were experimentally evaluated.

Finally, the Arcetri team is also involved in astronomical tests with SKA-Low demonstrators for early commissioning. Preliminary all-sky observations allowed us to perform sensitivity measurements and beam stability analysis of SKA-Low station prototype and compare them with numerical results and telescope specifications/requirements. Data analysis is ongoing to characterise the polarisation performance from astronomical observations.

The radio astronomy group at the Arcetri Astrophysical Observatory is involved in the study of new technologies for radio astronomy. One of these activities is related to “super-resolution” (SR). The concept of SR refers to various methods for improving the angular resolution of an optical imaging system beyond the classical diffraction limit. In optical microscopy several techniques have been successfully developed with the aim of narrowing the central lobe of the illumination Point Spread Function (PSF). In Astronomy, however, no similar techniques can be used.

A feasible and practical method to design telescopes with angular resolution better than the diffraction limit consists of using variable transmittance pupils. The simplest pupils are discrete binary phase masks with finite phase-jump positions, also known as Toraldo Pupils (TPs). In 2015 we have started a project devoted to a more exhaustive analysis of TPs, in order to assess their potential usefulness to achieve SR on a radio telescope by placing the TP at an exit pupil of the telescope.

Therefore, we have simulated and tested an optical module for K-band, based on standard low refractive index materials, able to narrow the PSF. We have then devised a method to interface this SR module with a test Cassegrain antenna and we have carried out extensive electromagnetic (EM) numerical simulations to study the performance of the complete optical system.

We have tested both the refractive and reflective optical systems using EM simulations, which have shown that when the SR module is mounted on the antenna one can effectively achieve a narrower main beam in the far-field. We have also started to carry out the first field-tests of the entire optical system.